Positron Emission Tomography (PET) has developed into an important research and clinical diagnostic tool providing non-invasive, in-vivo functional medical images. An important figure-of-merit for these systems is the spatial resolution of the images. In addition, although spatial resolution is an important parameter, it is often traded against other equally important parameters such as sensitivity and cost. This proposed innovative research program will develop planar silicon avalanche photodiode (APD) arrays that will have a strong positive impact on all of these important parameters. At present, there are no commercially available silicon APD arrays suitable for PET scanners. The PET topology places a severe burden on the avalanching device to have uniform characteristics over a one inch square area. A complete PET system also requires relatively uniform performance from tens to hundreds of arrays. By using advanced materials growth and processing techniques, the stringent materials requirements that have limited APD size and uniformity can now be surmounted. The planar silicon APD arrays to be fabricated on this Phase I SBIR program are targeted to display negligible cross talk, with greater than 80% gain uniformity across an array with greater than 80% average gain uniformity from array to array. PROPOSED COMMERCIAL APPLICATIONS: Planar silicon APD arrays can be quickly inserted into both clinical and animal PET systems. This technology can also be adapted to other nuclear scintillator applications such as high-energy physics calorimeters. Silicon APD arrays also have significant commercial and military applications in LIDAR and night vision systems.